In recent years, along with the development of technology for digitally recording moving pictures, still pictures, and other video data, large volumes of data are now being handled. CD or DVD and other optical disk devices are now in the limelight as large volume recording devices. Research for further increasing capacity is also underway.
FIG. 1 is a schematic view of a sectional structure of an optical disk of the CD-RW (rewritable) type and a method of emission of light.
Grooves 6 are provided in one surface of a light transmission type disk substrate 5 having a thickness D5 of about 1.2 mm. An optical recording layer 7 comprised of for example a dielectric film, a recording film, another dielectric film, a reflection film, etc. stacked in that order is formed on this surface. The film configuration and the number of layers are different according to the type and design of the recording material.
A protection layer 8 is formed above the optical recording layer 7.
An optical disk device for recording on or playing back a CD-RW type or other optical disk, in the normal configuration, has a light source for emitting light having a wavelength λ for recording or playback, an optical system including an object lens (condenser lens) having a numerical aperture NA for condensing the light emitted by the light source onto the optical recording layer of the optical recording medium, and a light receiving element for detecting light reflected from the optical recording layer, etc.
In for example the CD-RW system, as shown in FIG. 1, a laser beam LB for playback or recording is condensed by for example an object lens 50, passes through the light transmission type disk substrate 5, and is focused on the optical recording layer 7 of the optical disk having the above structure.
At the time of playback of the optical disk, return light reflected at the optical recording layer is received at the light receiving element, a predetermined signal is generated by a signal processing circuit, and a playback signal is taken out.
At the playback or recording of the optical disk, a spot size φ of the light on the optical recording layer is generally given by the following equation (1):φ=λ/NA  (1)
The spot size φ of the light directly affects the recording density of the optical recording medium. The smaller the spot size φ, the higher density of recording possible and the larger capacity possible. Namely, this means that the shorter the wavelength λ of the light or the larger the numerical aperture NA of the object lens, the smaller the spot size φ and the higher density recording possible.
For example, in the CD-RW system as shown in FIG. 1, in a configuration wherein the wavelength of the light source is in an infrared region (about 780 nm), the numerical aperture of the object lens is about 0.45, a phase change type recording layer is used for the recording layer, further the optical recording layer has a topography corresponding to the topography formed in the disk substrate 5, only the side of the optical recording layer having the topography close to the emitting side of the light for recording or playback, that is, the portions of the optical recording layer corresponding to projecting portions of the topography, is used as recording areas RA, and the side far from the emitting side of the light for recording or playback, that is, the portions of the optical recording layer corresponding to recessed portions of the topography, is not used as the recording areas RA, a recording capacity of about 700 MB is realized in the case of an optical disk having a diameter of 120 mm.
Regarding the above projecting portions and recessed portions, in the process of producing the master, in the surface of a master for forming the disk substrate 5 formed with the grooves the portions corresponding to the areas exposed by the laser beam or electron beam are referred to as “grooves G” and the areas lying between the grooves G are referred to as “lands L”. For example, in the case of the general process of production of the optical disk shown in FIG. 1, the projecting portions of the topography correspond to the grooves G, while the recessed portions correspond to the lands L.
Research is underway for further raising the density of optical disks. For example, document A “Optical disk recording using a GaN blue-violet laser diode” (Ichimura et al., Jpn. J. Appl. Phys., vol. 39 (2000), pp 937–942) proposes a technique for realizing a storage capacity exceeding 22 Gigabytes in an optical disk of a DVD size by using a blue-violet semiconductor laser and a 2-group object lens having a numerical aperture 0.85.
When the numerical aperture of the object lens becomes larger, the allowable tilt of the disk in an optical disk device is generally decreased. A comatic aberration W31 generated with respect to a tilt angle θ relative to the optical axis is given by the following Equation (2) according to document B “Aplanatic condition required to reproduce jitter-free signals in optical disk system” (Kubota et al., Appl. Opt., vol. 26 (1987), pp 3961–3973) and is roughly proportional to a cube of the numerical aperture NA and a thickness t of a protection layer (layer formed above the optical recording layer) of the optical disk. Note that, in Equation (2), n is a refractive index of the protection layer.W31=t(n2−1)n2 sin θ cos θ·NA3/2(n2−sin2θ)5/2  (2)
Accordingly, when the value of the permissible comatic aberration W31 is λ/4, in order to secure a allowable disk tilt equivalent to a DVD player in an optical disk device raised up to a numerical aperture of 0.85, it becomes necessary to make the thickness of the protection layer of the optical disk as thin as about 0.1 mm.
FIG. 2 is a schematic view showing a sectional structure of an optical disk formed by the technique reported in the above document A and the method of focusing light thereto.
Grooves 2 are provided in one surface of a disk substrate 1 having a thickness D1 of 1.1 to 1.2 mm. An optical recording layer 3 having a thickness D3 comprised of for example a reflection film, dielectric film, recording film, and dielectric film stacked in that order is formed on this surface. The film configuration and the number of layers are different according to the type and design of the recording material.
A light transmission type protection layer 4 having a thickness D4 of about 0.1 mm is formed above the optical recording layer 3.
In the above system, as shown in FIG. 2, the laser beam LB for playback or recording is condensed by the 2-group lens comprised by for example a first lens (close lens) 12 and a second lens (far lens) 14, passes through the light transmission type protection layer 4, and is focused on the optical recording layer 3 of the optical disk having the above structure.
At the time of playback, the return beam reflected at the optical recording layer is received at the light receiving element, a predetermined signal is generated by the signal processing circuit, and the playback signal is taken out.
In the method of production of the above optical disk, a stamper having grooves in its surface is formed by transfer from a disk master having grooves in its surface, the surface shape is transferred from the stamper to form a disk substrate 1 having grooves 2 in its surface, and an optical recording layer 3 comprised by the stack of for example a reflection film, dielectric film, optical recording layer, and dielectric film is formed by this film formation order. This is a reverse order to the ordinary order. Finally, the light transmission type protection layer 4 is formed above the dielectric film. By this technique, an optical disk having a protection layer of a thickness of 0.1 mm can be formed.
In the above system, in order to improve the planar recording density, a land and groove recording method wherein a depth D2 of the groove structures is made about λ/6n (λ: wavelength of light source of optical disk device, n: refractive index of light transmission type protection layer), the optical recording layer has a topography corresponding to the groove structures, and both areas of the side close to the emitting side of the light for recording or playback in this optical recording layer having topography, that is, the lands L, and the side far from the emitting side of the light for recording or playback, that is, the grooves G, are used as the recording area RA is employed.
In the land and groove recording system, the track pitch TP corresponds to the distance of the center position of a land L to the center position of a groove G and is specifically set to about 0.3 μm.
Details of the land and groove recording system are described in document C “Land and groove recording for high track density on phase-change optical disks” (Miyagawa et al., Jpn. J. Appl. Phys., vol. 32 (1993), pp 5324–5328) etc.
In this system, in order to make the signal amplitudes of the lands and the grooves equal, the grooves of the disk substrate are formed so that a ratio (duty ratio) of the widths of the lands and the grooves after the formation of the optical recording layer becomes about 1:1. The width of the grooves is determined so that the widths of the recording films corresponding to the lands and the grooves become equal when for example the width of the grooves formed in the disk substrate is made about 60% of the pitch of the grooves and the dielectric film, recording film, other dielectric film, and reflection film are stacked over the entire surface while covering the inside walls of the grooves.
Also, in order to reduce the amount of the light reflected from adjacent tracks, that is, the crosstalk component, using groove interference, the depth of the grooves is made λ/6 n.
When employing the above land and groove recording system, however, generally, when recording a signal at the side far from the emitting side of the light for recording or playback, that is, in the grooves, the phenomenon (crosswrite) of the signal marks recorded previously in the lands closer in being erased tends to easily occur.
This is due to the fact that the optimum emitted output at the time of recording is not always uniform since the numerical aperture of the object lens is large and the grooves are relatively deep and therefore electromagnetic waves are hard to be propagated in the grooves and the emitted output at the grooves is raised.
Further, this makes it difficult to achieve a uniform quality of the playback signals at the lands and the grooves.
Accordingly, a track density sufficiently making good use of the characteristic feature of the land and groove method, that is, the effect of cancellation of the crosstalk from adjacent tracks, has not been realized.
Also, since both of the lands and the grooves are used as recording areas, it was necessary to devise some means to secure compatibility with a read only (ROM: Read only memory) disks recording information by bits.